This invention relates generally to security and environmental control systems and more particularly to systems comprising multiple remote stations or panels interconnected by multiplexed communications with a master local station or panel via a common communications line.
Several different kinds of multiplex security systems have been previously employed. In general, such systems include a plurality of remote detectors whose outputs are connected over a common transmission line to a central receiver. The central receiver sequentially monitors the outputs of such detectors.
U.S. Pat. No. 3,927,404 to Cooper discloses a system in which remote detectors are interrogated one at a time by a control panel in a central receiver. An address pulse generator in the control panel transmits an address code to the remote monitors. Each monitor is provided with a counter-decoder which is responsive to a different address code. Upon interrogation, only the addressed monitor responds. Multiple local control panels can be interconnected and one of them operated as a central control facility controlling the other local panels and their respective remote panels.
This system has several principal drawbacks. First, data transmission is only one way--from the remote monitors to the control panel. Operation of the remote monitors cannot be altered by the central receiver. Second, polling priorities among remote monitors are determined solely by the control panel. Each remote detector is polled and responds whether it has information to send or not. Hence, considerable time can be wasted before a remote detector with an important message is polled. Third, the system requires transmission of serial address pulses. If 256 remote monitors are in the system, 256 address pulses must be sent and time provided for 255 monitors to respond before the 256th monitor can respond. Since the address generator operates at 51 Hz, this procedure requires 5 seconds--too slow for many applications. This system also lacks voice communications and means for selectively passing either voice or data through the local control panel to the central control facility over a single telephone line.
U.S. Pat. No. 3,828,313 to Schull, et al. shows another form of security system. That system comprises a central data processor connected to multiple remote receivers by a two-wire communications line and a clock line. The clock line provides for synchronous circuit operation. Two different binary message wave forms are used--one transmitted only by the central panel, the other transmitted by the remote panels. No means for prioritizing responses from remote terminals or for enabling such a terminal to interrupt with an emergency message are disclosed. It would be preferable if a single message format could be used throughout the system. It would also be preferable for the system to operate asynchronously.
Other security control systems are disclosed in U.S. Pat. Nos. 3,792,469; 3,803,594; 3,936,821; 3,938,118; 4,019,172; 4,032,908; 4,056,684 and 4,067,008. Many of these systems provide for multiplex communications between remote security panels or alarms and a control panel. However, none are known to provide asynchronous two-way communications in a format which permits serial polling of the remote terminals but prioritizes their responses so that the most urgent message is transmitted first. In addition, none are known to provide a remote terminal with means for signaling that it has an emergency message or a local terminal with means for acknowledging such a signal and thereby enabling the remote terminal to transmit its message. Also, none of the references disclose combined two-way voice-data communications between a local terminal and a central control facility over a single telephone line. Nor do they disclose means for transmitting voice communications from the remote terminal through the local terminal to central via a telephone line normally dedicated to data communications.
Accordingly, there remains a need for an improved multiplex security control system having these capabilities.